The invention relates to a method for machining the circumference of out-of-round workpieces, particularly piston rings and preferably piston rings in packets, having different out-of-round contours and rated diameters by series. More particularly the invention relates to a machining method in which, starting from a basic out-of-round contour with reference to a defined ring diameter, the angle of rotation of the piston ring packet to be processed can be varied so as to produce a defined out-of-round contour which deviates from the basic contour by a difference angle of rotation which leads or lags per revolution. The invention also relates to an apparatus operating according to the method and including a copying device in the form of a rotationally driven copying disc having a basic out-of-round contour as well as a mechanism composed of a dual lever for transferring the out-of-round contour from the copying disc to a tool holder mounted so as to be radially displaceable relative to the axis of the workpiece, with the copying disc as well as the workpiece packet being driven by synchronously running servomotors.
An apparatus for machining the circumference of piston ring packets having out-of-round outer and inner circumferences, as disclosed in German Patentschrift No. 1,043,015, comprises a tool holder in the form of a double rocker which is moved, through the intermediary of a sensing roller and a twin-lever mechanism, by a copying disc which rotates in synchronism with the piston ring circumference. In this apparatus, the copying device is designed so that a separate copying disc is required for each out-of-round contour and, when the twin-lever rocker is set to the piston ring diameter of a different series, the transmission ratio of the double lever inevitably remains unchangeable.
Based on the requirement that a piston ring, when installed in an engine, must lie against the cylinder wall with a defined radial pressure distribution and in a light slit tight manner, and based on the use of different piston ring materials because of extremely varied friction and thermal stresses, the major problem is the multitude of different out-of-round contours required for which a corresponding number of interchangeable copying discs must be held available.
Even if today these copying discs are calculated by electric computers, with respect to a certain basic diameter, by means of bending formulas for open rings, and the transfer of the calculated contours to the copying discs by NC controlled grinding machines is possible, the costs for manufacture and exchange of copying discs are not insignificant.
Frequently this circumstance results in the piston ring not being given the optimum configuration, but receiving the out-of-roundness of an existing copying disc which comes closest to the respective requirement.
Since various electronic control systems, such as, for example, the CNC control, have been developed in the course of progressive automatization of machine tools, the possibility exists of using such control systems when machining the circumferences of piston rings. Accordingly, the control values are fed from the memory of an electronic control to the setting and driving motors of the machine. Additionally, with the aid of appropriate measuring devices, the control values are constantly adapted by way of a comparison of the actual dimensions with the intended desired dimensions.
German Offenlegungsschrift (Laid-Open Application) No. 2,006,760 and German Patentschrift No. 2,732,354 relate to such out-of-round turning devices, in which the turning tool can be adjusted radially as a function of the angle of rotation of a workpiece which rotates together with the work spindle. The tool displacement for out-of-round machining of piston rings amounts to twice a full stroke from zero to a maximum value per revolution of the piston ring packet. Direct current servomotors and ball groove thread spindles, the latter being highly endangered by excessive wear, are used to displace the tool. Since the angular acceleration capability of an individual servomotor does not meet the requirements of modern metal cutting technology, attempts are being made to increase the acceleration capability by means of series or parallel connected servomotors.
German Patentschrift No. 2,427,826 relates to a device for the out-of-round turning by copying for numerically controlled lathes, with the rear end of the tool holder being supported in and by a cam which is rotatably mounted on an upper carriage and is included in the numerical control. The three-dimensional cam (copying discs), which is mounted in bearing blocks and is very difficult to manufacture, has not only one, but a plurality of out-of-round contours, corresponding practically to a plurality of juxtaposed copying discs. Although the rotary drive for the cam may be provided with a fixed, adjustable transmission ratio with respect to the revolution of the workpiece, such as, e.g. a ratio of 1:1, 1:2, etc., the cam rotates always at the same angular velocity.